Rainwater
harvesting involves the use of captured rainwater, usually from a
roof catchment, which otherwise would have soaked into the
ground, evaporated or entered the drainage system. Once captured,
the water can be drawn on for a variety of uses from irrigating
crops or gardens, as toilet flush water, in water features and
occasionally as a source of drinking water. Watering a garden with
rainwater collected in a water butt is a rudimentary form of
rainwater harvesting.
Where there is negligible potential human contact the rainwater
will usually only require coarse filtration to prevent leaf litter,
debris and small animals entering the system. If the rainwater is
to provide a potable water supply, thorough treatment is required
which makes this use uncommon.
During rainfall events the first 'flush' of water usually has
the lowest water quality due to contamination from leaf litter,
bird droppings and wind-blown pollutants that have adhered to the
roof surface or guttering. For this reason, many rainwater
harvesting systems divert the 'first flush' of water so that it is
not used.
The amount of rainwater that can be harvested is a function of
rainfall received and plan roof area. For example in Northern
Ireland where 2004 Annual rainfall was just over 1000mm/year, a
home with a 100m2 plan roof area could have 'harvested'
60m3 of rainwater, assuming that 60% of rain that falls
on a roof catchment is collected and used.
As well as capturing rainwater from roof catchments, rainwater
harvesting systems are also being designed in conjunction
with SuDS (Sustainable
Drainage Systems) to capture the rainwater which infiltrates
through permeable paving surfaces. The only addition needed to the
rainwater harvesting system is an oil trap to remove oils from the
paving surface (e.g. from cars) which are washed down into the
system.
In the UK there are corporation tax incentives available to help
with the cost of rainwater system investments.
In Germany, a study into rainwater harvesting by the
consultants Mall GmbH has shown that 35% of new buildings
built in Germany in 2005 were equipped with a rainwater collection
system. Turnover from the industry is worth 340 million euros and
5000 jobs have been created.
Case Studies
Read about rainwater harvesting at
the Eden Project in Cornwall
